This paper reviews some recent developments in the use of contactless modulation spectroscopy techniques for the in-situ characterization of the growth and processing of semiconductors. Photoreflectance (PR) measurements at 600°C on GaAs and Ga0.82A1 0.08As have demonstrated the potential of this method for in-situ monitoring during growth. Investigations PR, electron beam electroreflectance and differential reflectometry have shown that post growth (processing) information can be obtained about very thin Ga1AlxAs/GaAs epitaxial films, Ga Al As alloy composition, deep trap states, surface electric fields and carrier concentrations, lattice-mismatched strain, ion-implantation and annealing, and sputtering. In addition, characterization of semiconductor heterostructures can be performed.